1. Field of the Invention
This invention relates generally to electronic sensors for monitoring both temperature changes and sound using a single transducer, and more particularly to the construction of such sensors that more readily allows the temperature signal to be separated from the sound signal using conventional signal processing techniques (filtering).
2. Discussion of the Prior Art
In the Stasz U.S. Pat. No. 5,311,875, which is hereby incorporated by reference, there is described a system for electronically monitoring breathing patterns. The system described therein utilizes a plastic film exhibiting both pyroelectric and piezoelectric properties such as a polyvinylidene fluoride (PVDF) film. The film transducer has a conductive electrode on opposed major surfaces thereof and electrical wires connect the electrodes to an electronics module that is designed to separate the transducer output into two separate channels, one being for temperature and the other being for sound or vibration.
Our testing has shown that the signal proportional to temperature variation greatly exceeds that due to noise or vibration by a factor of about 150:1. The temperature-related signal also exhibits a substantially faster response time than the sound signal and is rich in frequency components in the 20 Hz to 50 Hz range. These facts result in a difficulty in preventing the thermal component of the transducer from crossing over into the sound channel, often leading to the occurrence of false positives when both sound signals and temperature signals are being simultaneously monitored.
The problem thus presents itself as to how one may effectively separate the signals when the frequency components of each signal overlap. Resorting to a signal processing approach has proven difficult.
We have found that by modifying the mechanical construction of the pyro/piezo transducer to increase its thermal mass, the effective rise time of the thermal signal is greatly reduced, while not seriously attenuating the sound/vibration-related signal amplitude. Decreasing the rise time of the thermal signal results in a shift of the frequency components thereof so as not to overlap with the frequency components in the sound channel. As such, conventional filtering steps can be performed to produce separate, isolated channels.
In accordance with the present invention, control over the thermal mass of the transducer described in the aforereferenced Stasz patent is achieved by affixing a layer of plastic foam material onto the transducer film on at least one major surface thereof. Thus, in accordance with a preferred embodiment of the present invention, the combination thermal and vibration sensor may comprise a PVDF film of a predetermined shape configuration with first and second major surfaces. A thin layer of a conductive material is deposited upon the first and second major surfaces of the film and electrical leads are attached at one end to the electrodes. An outer moisture impervious layer is adhered to the electrode on one side of the film and a foam layer of a predetermined thickness and coextensive with the area of the PVDF film is bonded to the opposite major surface. The thickness of the foam layer is directly related to the rise time of the thermal signal. Lowering the rise time of the thermal signal effectively shifts the frequency components comprising the thermal signal so as to no longer overlap with the predominant frequency components of the piezo or sound signal. Now, conventional filtering techniques can be used to separate the two channels.